Transmitting several frequencies from a single broadband antenna
system requires the use of a combining system, or combiner, composed of
RF filters and interconnecting transmission line. Generally, a combiner
can be categorized as branched (star point) or balanced
(constant-impedance). These types may use band-reject (notch) or
Components of combiners
There are several ideas to know when discussing combiners.
Tee or Star-Point Junctions. A tee junction, shown in Figure
1, is a coaxial component that provides for two RF signals to flow into
a common path; a star-point junction is a tee with more than two input
paths. This basic coaxial component is one of the building blocks of a
Figure 1. Tee junction schematic.
Filters. Filters sort RF frequencies, attenuating some while
allowing others to pass readily. Depending on the design, a filter may
either attenuate (band-reject type) or pass (band-pass
type) a narrow bandwidth.
Frequency Response. Energy transfer through the band-pass
filter is highest, or least attenuated, at the resonant frequency, and
drops off at frequencies above and below that frequency. This
frequency response is the fundamental property that enables a
filter cavity to sort frequencies.
Group Delay. The signal takes a finite amount of time to pass
through the cavity, and just as more energy is lost, so more time is
taken at non-resonant frequencies. This is termed group delay
difference, or group delay for short. Excessive group delay within
the pass band results in signal distortion.
Figure 2. Branched combiner schematic.
Branched or Star Point Combiners. A branched combiner is a
simple combination of a specially tuned tee or star point junction and
the required number of filters to ensure a sufficient amount of
isolation. For example, an FM branched combiner containing a
three-cavity band-pass filter in series (Figure 2) may be used to
provide the high-quality isolation required for two frequencies more
than 1.6MHz apart.
The three-cavity filter system in Figure 2 is tuned for one of the
channels in a two-station branched combiner and forms one leg of the
combiner. The second leg is a similar combination of filters tuned for
the other frequency. Figure 3 shows the basic combiner
TX1 and TX2 are the signals from transmitters 1 and 2 as they enter
the combiner. The length of the coaxial line between each set of
filters and the tee junction is adjusted to provide a high impedance
(approaching an open circuit) to the other frequency, so that the power
flow of each signal is through its own filter, out of the tee junction,
and up to the antenna.
In a band-pass balanced combiner system, band-pass filters are used
within the hybrid ring. The basic system layout is similar to that of a
Figure 3. Band-pass filter balanced
The power flow is shown in Figure 3. In the notch system, the
filters reflect signal TX1 entering port 1. In the band-pass system TX1
also enters port 1, but passes through the hybrid ring's band-pass
filters and out port 4, while signal TX2, entering at port 3, is
reflected by the filters and exits at port 4.
Bob Surette is manager of RF engineering of Shively Labs,